Adrenocorticotropic hormone (ACTH) and α,β and γ-melanocyte stimulating hormone (α,β and γ-MSH) are generally referred to as melanocortin peptides or melanocortins for their common origin as posttranslational derivatives of pro-opiomelanocortin (POMC).
α-MSH is a tridecapeptide that exerts pleiotropic effects in several physiological pathways including modulation of fever and inflammation, control of feeding behaviour and energy homeostasis, control of autonomic functions and increase in melanogenesis. Researches explored the idea that α-MSH might exert beyond the established anti-inflammatory activity also anti-infective activity. Thus, α-MSH was found to have a potent antimicrobial activity, especially against Candida albicans and Staphylococcus aureus. Candida albicans, in particular, belongs to a group of fungi which may cause severe infections, i.e. candidosis, also comprising, for instance, C. albicans, C. glabrata, C. krusei, C. Lusitaniae, C. stellatoidea, C. tropicalis, C. parapsilosis, C. pseudotropicalis, C. guilliermondii. Under physiological conditions, said ubiquitous saprophytes are natural and inoffensive hosts of the health organism mucosa. However, when the natural defences, mainly represented by immunologic system, are weakened because of stress, pharmaceutical therapies, like with antibiotics, cortisone, immunosuppressant, cytostatic agents, contraceptives, or even radiotherapy, surgery interventions requiring insertion of a catheter or long lasting infusions or dialysis, inserted prosthesis, illnesses such as diabete, obesity or malnutrition, hyperthyroidism, tumors, or other events like burnings or trauma, especially in the infant or in elder patients, Candida may become a dangerous infective agent. In particular, infections may involve the skin, the respiratory tract, including mouth, throat and pharynx, the digestive apparatus or it may even cause infections to the urogenital apparatus or systemic infections to the cardio circulatory system, possibly causing endocarditis.
The analysis of the Melanocyte Stimulating Hormone peptide demonstrated that the sequence (6-13) contains the the invariant core sequence His-Phe-Arg-Trp (6-9) (SEQ ID NO 17), which is common to all melanocortin peptides and is responsible for the binding to the melanocortin receptors. Its C-terminal tripeptide Lys-Pro-Val (11-13) has been identified as relevant both for receptor activation and for the antimicrobial activity through the increase in cAMP. On the contrary, the N-terminal was not identified as having an influence on candidacidal activity. Further studies, such as, for instance, the alanine scan, indicate that the proline residue in position 12 has an important role for such activity.
In addition, Lys-11 and Pro-12 proved to be essential for the antimicrobial activity, in fact, their substitution causes the activity to be nearly abolished.
On the other hand, substitution in position 7 of the core sequence may enhance the potency of melanocortins. A substitution in position 7, such as, for instance, the replacement with D-Nal, increases the affinity of α-MSH for the melanocortin receptor MC4R. Substitution at position 12 with a more lipophilic amino acid, like Phe, resulted in a marked increase in candidacidal activity.
In spite of all the peptides already tested, there is still an unsatisfied need in the prior-art for more active compounds, which would cause less side effects to patients, thus resulting more tolerable, and which would also be safer and easier to be administered to patients as well as more stable, thus allowing the industrial preparation.